a=int(input())
M=0
a1=a
a2=a//10
a3=a//100
a4=a//1000
a5=a%1000
a6=a%1000//10
a7=a%1000//100
a8=a%100
a9=a%100//10
a10=a%10
S=[a1,a2,a3,a4,a5,a6,a7,a8,a9,a10]
if 999<a<10000:
    for y in S:
        k=0
        for i in range (1,y//2+1):
            if y%i==0:
                k += 1
        if k ==1:
            M=max(M,y)
print(M)

